1. Field of the Invention
The present invention relates to a semi-reflective film and a reflective film for an optical information recording medium which has a high cohesion resistance, high light resistance, and high heat resistance simultaneously with a high reflectivity, high transmissivity, low absorptivity, and high thermal conductivity in the field of optical information recording media such as compact disc (CD), digital versatile disc (DVD), Blu-ray Disc, and HD DVD. It also relates to a sputtering target for an optical information recording medium which is used in depositing the semi-reflective film or reflective film; and an optical information recording medium having the semi-reflective film or reflective film.
2. Description of the Related Art
Optical information recording media (optical discs) include various types, and the three main types categorized by the writing/reading system are read-only, write-once, and rewritable optical discs. In order to increase storage capacity, single side, multilayer optical discs have also been developed from the ordinary single side, single layer optical discs. For example, in the case of a single side, dual layer optical disc wherein the signals are written and read in the recording layer which is farther from the side of the laser beam incidence, the laser beam should be transmitted through the recording layer which is nearer to the laser beam incidence, reflected by the farther recording layer, and again transmitted through the recording layer which is nearer to the laser beam incidence. Accordingly, a semi-reflective film capable of reflecting and transmitting the laser beam is used for the recording layer which is nearer to the laser beam incidence.
Materials which function as a semi-reflective film include metals such as Ag, Al, Au, Pt, Rh and Cr and elemental semiconductors such as Si and Ge. Among these, pure Ag and Ag alloys containing Ag as their main component are featured in view of (1) high light efficiency (i.e., the total of reflectivity and transmissivity), (2) high reflectivity for the blue-violet laser (wavelength: 405 nm) used in Blu-ray Disc and HD DVD, and (3) high thermal conductivity which enables adequate diffusion of the heat generated in the recording film upon recording of the signals. Such Ag based materials exhibit excellent properties for use as a semi-reflective film of an optical disc including high reflectivity, high transmissivity, low absorptivity, and high thermal conductivity. In this connection, the absorptivity is determined according to the following equation: absorptivity=100%−(reflectivity+transmissivity). However, in order to accomplish sufficient function as a semi-reflective film of an optical disc with long term reliability, the Ag based materials need to overcome the disadvantages of the Ag based materials, namely, (1) cohesion resistance, (2) light resistance and (3) heat resistance.
[1] Cohesion Resistance
Ag based materials are likely undergo cohesion under the effect of heat and/or a halogen such as fluorine, chlorine, bromine or iodine. When it is kept under high temperature, high humidity conditions used in the reliability test of an optical disc, or arranged in contact with a halogen-containing organic material typically of the organic dye recording film, the protective layer or the adhesive layer, cohesion may take place, resulting in the increase in the surface roughness of the thin film or loss of the continuity of the thin film, which may in turn invite loss of the function of the material as a semi-reflective film or a reflective film.
[2] Light Resistance
For example, a single side, dual layer, read-only optical disc has a basic cross sectional structure of polycarbonate (PC) substrate\semi-reflective film\adhesive layer\reflective film\PC substrate. When such an optical disc is irradiated by a Xe lamp (a lamp having a spectrum resembling that of the sunlight) in a so called “light resistance test”, the semi-reflective film experiences decrease in the reflectivity when the film comprises a Ag based material, and in such a case, reading of the signal becomes impossible once the reflectivity is reduced to below the threshold value that is required for detecting the signal to be read.
[3] Heat Resistance
A single side, dual layer, write-once optical disc, for example, has a basic cross sectional structure of PC substrate\recording film\semi-reflective film\spacer\recording film\reflective film\PC substrate, and a single side, dual layer, rewritable optical disc has a basic cross sectional structure of PC substrate\dielectric and protective layer\interface layer\recording film\interface layer\dielectric and protective layer\semi-reflective film\intermediate layer\dielectric and protective layer\interface layer\recording film\interface layer\dielectric and protective layer\reflective film\PC substrate. In the case of recordable optical discs including such write-once and rewritable optical discs, the recording layer is heated to a temperature as high as 300° C. to 600° C. during the writing, and a very severe thermal hysteresis is applied to the semi-reflective film or the reflective film. The growth of the crystal grains of the thin film and loss of the continuity of the thin film caused as a result of such thermal hysteresis impair the function of the semi-reflective film and the reflective film.
Various attempts have been made to improve the pure Ag, mainly by alloying the Ag. For example, corrosion resistance is improved in U.S. Pat. No. 6,007,889 by adding Au, Pd, Cu, Rh, Ru, Os, Ir or Pt to Ag; in U.S. Pat. No. 6,280,811, Published Japanese translation of PCT international publication for patent application 2002-518596, by adding Au, Pd, Cu, Rh, Ru, Os, Ir, Be or Pt to Ag; and in U.S. Pat. No. 5,948,497, Japanese Patent Application Laid-Open No. 06-208732 by adding Pd or Cu to Ag. The inventors of the present invention have also disclosed in Japanese Patent No. 3365762 a method for improving crystal structure stability by adding Nd to Ag wherein the crystal structure stabilized by suppressing Ag diffusion and crystal grain growth.
In spite of such efforts, no Ag based alloy has been found that exhibits high reflectivity, high transmissivity, low absorptivity and high thermal conductivity simultaneously with high cohesion resistance, high light resistance and high heat resistance, and accordingly, there is a strong demand for a Ag based alloy which satisfies requirements for all these properties.